1. Field of the Invention
The present invention is related to the field of materials handling technology and concerns a method and a device for stacking flat objects, in particular printed products such as newspapers, periodicals, etc., that are supplied to be stacked serially in a supply stream.
2. Description of Related Art
Printed products are stacked, for example, for intermediate storage between processing steps or for packaging prior to being dispatched. Serving for the stacking, for example, is a stacking shaft with a substantially vertical stacking axis, which stacking shaft has a ground plan essentially corresponding to the shape of the objects to be stacked and comprises at its bottom an advantageously vertically displaceable stacking table. On its upper end (opposite the stacking table) the stacking shaft is open. The products are transported to this upper shaft end and are positioned in the stacking shaft through its opening.
Printed products are transported to a stacking shaft of the above described kind arranged in an imbricated stream loosely lying on a conveying surface, for example, on a conveyor belt, wherein the leading product edges are situated on the top side of the stream. The products are conveyed in the named manner to the opening of the stacking shaft, and every product is pushed over this opening until it drops into the shaft or onto products already stacked in the shaft, respectively. In order to reduce the uncontrolled fall to a minimum, the stacking table, as already mentioned, is usually displaceable in a vertical direction and is adjusted such that the upper side of the stack being produced in the shaft is located immediately below the shaft opening at all points in time. This, however, signifies that every product when pushed over the stacking shaft opening, slides over the stacking table or over the top one of the products stacked in the stacking shaft. This sliding movement, particularly for not very rigid products, constitutes a movement that may not be fully controllable and, for this reason, may give rise to problems.
Stacking methods using a stacking shaft being supplied by an imbricated stream of loosely lying objects are described, for example, in the publications DE-2752513 (or CA-1091707) or EP-0309745 (or U.S. Pat. No. 4,886,265).
For stacking flat products with little rigidity it is also proposed to supply the products to the stacking shaft by means of grippers coupled to a traction organ, each one of the grippers holding one product by its leading edge and pulling it over the stacking shaft opening and then releasing it. Depending on the position of the upper side of the stack in the stacking shaft, a product being released by the gripper will drop into the shaft or, being pulled by the gripper, it will slide across the upper surface of the stack. Stacking methods of this type are described, for example, in the publications EP-0059746 (or U.S. Pat. No. 4,666,143) or DE-3130945 (or U.S. Pat. No. 4,445,681).
Both stacking methods briefly described above include horizontal supply of the products and free fall into the stacking shaft, i.e. a change of supply direction that is difficult to control and a free fall that can hardly be controlled. Even if the free fall is reduced to a minimum, the horizontal feeding-in still has to be sharply braked and the product has to be slid onto the stack surface (being pushed or pulled), which is not desirable, particularly for thin and not very rigid products.
The invention sets itself the objective of creating a method and a device for stacking flat objects being supplied serially in a supply stream, in particular printed products, without free fall and without sliding movement or at least with significantly reduced sliding movement such that stacking such objects, which are known as being difficult to stack, becomes easy. The method and device are to be easily utilized not solely for producing simple stacks, but also for producing so-called cross stacks, in which groups of superimposed products are rotated relative to one another by 180xc2x0 around the stacking axis.
The main steps of the method according to the invention are the following: supplying the objects to the stacking system each held gripped individually, forming of each supplied group of still individually held objects a stack-like arrangement, stabilizing the stack-like arrangement and only then releasing the objects from being individually held and thereby forming the desired stack.
The objects are supplied individually held gripped in the zone of one of their edges, and in groups, wherein every group comprises a plurality of objects being oriented in parallel to one another, substantially transverse to the conveying direction, aligned with one another, and with such a small spacing between one another that they can be arranged stack-like while each one of the objects is still held gripped. The objects of each group forming a stack-like arrangement are then stabilized.
In the named stack-like arrangement zones opposite the held edges of the objects of the group are positioned relative to one another in substantially the same way as in a real stack, this means they are directly adjacent to one another and aligned with one another to the greatest possible degree, while the zones of the held edges are aligned with one another but spaced from one another due to the means holding them gripped.
This stack-like arrangement is stabilized in a suitable manner and only then the objects are released from being held gripped, so that the zones of the originally held gripped edges come to lie adjacent to one another and the group of objects forms a real stack, in which the objects are superimposed or juxtaposed and aligned with one another. Conveying away the stacks is realized in any known manner.
Establishing the stack-like arrangement constituting an intermediate phase of the method according to the invention is only possible if the flat objects are bendable at least to a limited extent at least parallel to their held edge and if the distances between the objects defined by their held conveyance is matched to this ability to bend and to the number of objects to be processed within one group. Completely rigid objects can theoretically only be arranged in a stack-like arrangement, as described above, if their held gripped edges are adjacent to one another without any spacing in between. A corresponding held conveyance, however, is rather difficult to realize.
The degree to which the object zones opposite the held edges are able to be aligned with one another in the stack-like arrangement is also dependent on the spacing between the held edges and on the manner in which the held edges are aligned with one another. In an arrangement in which the held edges of a group form a curved line, in particular a circular arc, there is a higher degree of alignment of the object zones opposite the held edges than in a straight-line arrangement of the held edges.
For producing cross stacks of objects which are supplied all having the same orientation, e.g. every second stack-like arrangement of a group of objects is rotated, wherein rotated and not rotated groups are stacked upon one another as crossed layers of the cross stack.